The invention relates to a semiconductor device having a semiconductor body comprising a substrate region of a first conductivity type, a surface adjoining layer-shaped semiconductor region provided on said substrate region, at least a part of said layer-shaped semiconductor region being of the second conductivity type and forming a p-n junction with said substrate region, an island-shaped part of said layer-shaped semiconductor region being bounded laterally by a separation region which extends from the surface over substantially the whole thickness of the semiconductor region, at least one zone of a semiconductor circuit element being provided within said island-shaped part, in which the overall net doping of the second conductivity type of said layer-shaped semiconductor region in atoms per unit of surface area is so small that upon applying a voltage in the reverse direction across said p-n junction the depletion zone within the island-shaped part extends at least locally from said p-n junction up to the surface at a voltage which is lower than the breakdown voltage of the p-n junction.
The expression "doping of a given conductivity type" should be understood herein in the algebraic sense. For example, an n-type region has a positive n-type doping concentration but a negative p-type doping concentration.
It is furthermore to be noted that in the operating condition the same reverse voltage need not be present at any point across the said p-n junction, for example, as a result of currents flowing parallel to the surface. As a result of the voltage drop caused by such currents it may occur, for example, that the island-shaped region is depleted entirely up to the surface in places where the reverse voltage is high and is not fully depleted up to the surface in places where the reverse voltage is lower. It is essential that depletion should occur throughout the thickness of the island-shaped region in those places where the surface field strength is high.
Semiconductor devices of the kind described (so-called "RESURF" devices, "RESURF"="REduced SURface Field") are known from Philips Journal of Research, Volume 35, No. 1, 1980, pp. 1 to 13. Such devices are also described in the Dutch Patent Applications Nos. 7800582, 7807834 and 7807835 of Applicants open to public inspection and corresponding to U.S. Pat. Nos. 4,233,617 and 4,292,642, which are hereby incorporated in this Application by reference. In these known devices the layer-shaped semiconductor region is doped substantially homogeneously.
As is explained in the cited Philips Journal of Research publication, the breakdown voltage of p-n junctions in semiconductor devices of the kind described may be very high and may closely approach or may even equal the unidimensionally computed value (at which the p-n junction is considered to be flat and infinitely extended). This is a result of the fact that, at high reverse voltage across the p-n junction, the field strength at the surface is considerably reduced by the complete depletion of the layer-shaped region.
Since the net doping of the layer-shaped region must be comparatively small, however, such semiconductor devices have a rather low current-carrying capacity via the homogeneously doped layer-shaped region. An increase of the doping concentration cannot provide a solution in this case since then even at high voltage the layer-shaped region can no longer be depleted entirely, so that the breakdown voltage of the p-n junction would be reduced.
Another disadvantage of the known device described above is that, when an active zone of the first conductivity type is present in the island-shaped region (for example, the base zone of a transistor), expansion of the depletion zone from the substrate region up to said active zone ("punch-through") can occur in certain circumstances. This applies in particular to the use in emitter follower applications.
In addition, the known devices described are often difficult to manufacture in a reproducible manner, since variations in the thickness and the doping of an epitaxial layer easily occur and may have an important influence on the electric characteristics.